quot: Make nusers unsigned

Approved by: re (cperciva)
Fixes: 195b00ec45e5 ("quot: Clean up")
Reviewed by: rlibby
Differential Revision: https://reviews.freebsd.org/D53224

(cherry picked from commit fa347930f89d4f9d1956573c3bd4b5987127bc12)
(cherry picked from commit 2427ae41b7cf11e75f9a80d6c274a09eeb9ae979)